Cellular automata simulations on nanocrystallization processes: From instantaneous growth approximation to limited growth

Cellular automata simulations have been performed to simulate the crystallization process under a limited growth approximation. This approximation resembles several characteristics exhibited by nanocrystalline microstructures and nanocrystallization kinetics. Avrami exponent decreases from a value n = 4 indicating interface controlled growth and constant nucleation rate to a value n ~ 1 indicating absence of growth. A continuous change of the growth contribution to the Avrami exponent from zero to 3 is observed as the composition of the amorphous phase becomes richer in the element present in the crystalline phase.Ministerio de Ciencia e Innovación de España (MICINN) y Unión Europea FEDER. MAT2010-20537Junta de Andalucía. Procesos Asistenciales Integrados (PAI) FQM-646

Nanocrystalline Fe-Nb-(B,Ge) alloys from ball milling: microstructure, thermal stability and magnetic properties

Fe75B20Nb5, Fe75Ge10B10Nb5 and Fe75Ge20Nb5 alloys were prepared by ball milling from pure powders and their microstructure and magnetic properties were studied. A nanocrystalline solid solution of-Fe type is the main phase formed, although traces of some intermetallics were found in the Fe-B-Nb alloy. The local arrangements of Fe atoms in Ge containing alloys continuosly evolves with milling time. The obtained powders are thermally stable even heating up to 773 K. After heating up to 1073 K, intermetallic compounds are detected. The best soft magnetic properties are achieved after heating up to 773 K, due to stress relaxation of the nanocrystalline microstructure (for Fe-Ge-Nb alloy, coercivity ~600 A/m

On the isothermal kinetics analysis of transformations in metastable systems. A combined use of isothermal and non-isothermal calorimetry

A procedure to optimize the isothermal calorimetric data of very slow transformation processes of metastable systems is proposed. The method uses an experimental baseline to identify the transitory effects due to the equipment. Moreover, the combined use of isothermal and non-isothermal results is shown to be effective to overcome the intrinsic problems of low signal and signal drift for such processes. The procedure has been applied to the analysis of the nanocrystallization kinetics of the Fe60Co18Nb6B16 alloy at different devitrification stages. Based on microstructural observations, an instantaneous growth approach was assumed and a phenomenological expression of the dependence of the nucleation frequency with both the transformed fraction and the temperature was obtaine

Evidence of spin disorder at the surface–core interface of oxygen passivated Fe nanoparticles

Hysteresis, thermal dependence of magnetization, and coercivity of oxide coated ultrafine Fe particles prepared by inert gas condensation and oxygen passivation have been studied in the 5–300 K range. The results are found to be consistent with a spin-glasslike state of the oxide layer inducing, through exchange interaction with the ferromagnetic core, a shift of the field cooled hysteresis loops at temperatures below the freezing at approximately 50 K.Dirección General de Investigación y Desarrollo. Gobierno de España-CICYT MAT95- 1042-C02-02Dirección General de Investigación y Desarrollo. Gobierno de España-PB96-0863-C02-02.Comisión Europea-ERBFMBI-CT95-0534

Nanostructuring as a procedure to control the field dependence of the magnetocaloric effect

In this work, the field dependence of the magnetocaloric effect of Gd bulk samples has been enhanced through nanostructuring of the material. Nanostructuring consists in multilayers preparation by alternative rf-sputtering deposition of Gd layers and Ti spacers onto glass substrates. The results obtained for the multilayers were compared to those obtained for the Gd bulk. Assuming a power law for the field dependence of the magnetic entropy change (ΔSM ∝ Hn ), higher field dependences close to the transition in a wider temperature range are obtained for the multilayer material (n = 1.0) with respect to the bulk counterpart (n = 0.78). The effect of a Curie temperature distribution in the multilayer material (due to variations of the layer thickness) has been studied through numerical simulations to explain the observed field dependence of the magnetocaloric effect, obtaining a remarkable agreement between experiments and results.Ministerio de Economía y Competitividad español y EU-FEDER. MAT2013-45165-P y MAT2016-77265-RMinistry of Education and Science of the Russian Federation. Project No. 258